Nonaqueous electrolyte secondary cells

ABSTRACT

A nonaqueous electrolyte secondary cell comprises a rolled-up electrode unit  2  composed of a positive electrode  23 , a negative electrode  21  and a separator  22  interposed therebetween, and a negative electrode current collector plate  3  and a positive electrode current collector plate  30  joined to the respective ends of the electrode unit  2 . The negative electrode collector plate  3  is joined to an edge of the negative electrode  21  projecting at one of the opposite ends of the electrode unit  2 . The collector plate  3  has a two-layer structure comprising a copper layer  31  made of copper or an alloy consisting predominantly of copper, and a metal layer made of a metal not forming an intermetallic compound with lithium and having a lower laser beam reflectivity than copper or an alloy consisting predominantly of the metal. The collector plate  3  has its copper layer  31  contacted with the edge of the negative electrode  21  and welded thereto with a laser beam. This improves the weldability of the collector plate  3  to the rolled-up electrode unit  2  to achieve a high current collecting efficiency.

[0001] This application is a division of application Ser. No.09/636,506, filed Aug. 10, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to nonaqueous electrolyte secondarycells, such as cylindrical lithium ion secondary cells, which comprisean electrode unit encased in a battery can and serving as an electricitygenerating element and which are adapted to deliver the electricitygenerated by the electrode unit to the outside via a positive terminalportion and a negative terminal portion like. The invention relates alsoto processes for fabricating such cells.

BACKGROUND OF THE INVENTION

[0003] Nonaqueous electrolyte secondary cells of the type mentionedcomprise a rolled-up electrode unit formed by laying a positiveelectrode and a negative electrode, each in the form of a strip, overeach other in layers with a separator interposed therebetween androlling up the resulting assembly into a spiral form. The rolled-upelectrode unit is encased in a battery can.

[0004] The electric power generated by the rolled-up electrode unit isdelivered to the outside through an arrangement including a plurality ofconductive current collector tabs having their base ends attached toeach of the positive electrode and the negative electrode of theelectrode unit. The positive current collector tabs extending from thepositive electrode have outer ends connected to a positive terminalportion, and the negative current collector tabs extending from thenegative electrode have outer ends connected to a negative terminalportion. This arrangement is widely used.

[0005] However, the current collecting arrangement comprising aplurality of collector tabs has the problem of failing to achieve asufficient current collecting effect when used in nonaqueous electrolytesecondary cells of large size having a high current value since the cellhas increased electrode areas although producing a satisfactory currentcollecting effect in nonaqueous electrolyte secondary cells of smallsize which are relatively low in current value.

[0006] Further the connection of the current collector tabs to eachelectrode terminal portion requires a complex structure and complicatedprocedure, hence the problem of low work efficiency or productivity.

[0007] Accordingly, a cylindrical nonaqueous electrolyte secondary cellhas been proposed which has a current collecting structure comprising anegative electrode current collector plate 36 and a positive electrodecurrent collector plate 30 as shown in FIG. 7. This cell has a batterycan 1 formed by a cylinder 15 and lids 16, 16 secured to opposite openends of the cylinder. A rolled-up electrode unit 2 is enclosed in thebattery can 1. The negative electrode collector plate 36 and thepositive electrode collector plate 30 are arranged at respective ends ofthe electrode unit 2 and joined to the unit 2 by laser welding. Thecollector plates 36, 30 are connected by lead portions 37, 34respectively to a negative terminal assembly 4 and a positive terminalassembly 40 mounted on lids 16, 16.

[0008] The rolled-up electrode unit 2 comprises a positive electrode 23,separator 22 and negative electrode 21 each in the form of a strip. Thepositive electrode 23 is formed by coating a current collector ofaluminum foil with a positive electrode active material. The negativeelectrode 21 is formed by coating a current collector of copper foilwith a negative electrode active material.

[0009] The positive electrode 23 and the negative electrode 21 are eachsuperposed on the separator 22, as displaced from the separatorwidthwise thereof and rolled up into a spiral form, whereby the edge ofthe positive electrode 23 is positioned as projected outward beyond theedge of the separator 22 at one of opposite ends of the electrode unit 2in the direction of its winding axis, and the edge of the negativeelectrode 21 is positioned as projected outward beyond the edge of theseparator 22 at the other end of the unit 2. The positive electrodecurrent collector plate 30 is made of aluminum, and the negative currentcollector plate 36 is made of copper.

[0010] With the current collecting structure wherein the collectorplates 36, 30 are joined to the respective ends of the electrode unit 2as described above, the collector plates can be welded to the unit 2contactlessly without applying pressure to the plates for welding. Thisachieves an improved work efficiency or productivity.

[0011] The process for fabricating the nonaqueous electrolyte secondarycell shown in FIG. 7, however, has the problem that when the negativeelectrode collector plate 36 is disposed at and welded to the edge ofthe negative electrode 21 of the unit 2, sufficient energy can not begiven to the portion to be welded since the copper forming the collectorplate 36 has high reflectivity for the laser beam used for welding,forming a faulty weld and increasing the electric resistance between theunit 2 and the negative electrode collector plate 36 to result in animpaired current collecting efficiency. If the collector plate 36 ismade from nickel, the weldability of the plate 36 to the electrode unit2 can be improved, whereas the collector plate 36 of nickel has greaterelectric resistance than the plate 36 of copper and therefore exhibits alower current collecting efficiency.

[0012]FIGS. 20 and 23 show another conventional nonaqueous electrolytesecondary cell, which comprises a cylindrical battery can 1 including acylinder 15 and lids 16, 16 welded to respective opposite ends of thecylinder, and a rolled-up electrode unit 5 enclosed in the can 1. A pairof positive and negative terminal assemblies 110, 110 are mounted on therespective lids 16, 16 and each connected to the electrode unit 5 by aplurality of electrode tabs 6 for delivering the electric powergenerated by the unit 5 to the outside through the terminal assemblies110, 110. Each lid 6 is provided with a gas vent valve 13 which isopenable with pressure.

[0013] As shown in FIG. 22, the rolled-up electrode unit 5 comprises apositive electrode 51 and a negative electrode 52 each in the form of astrip and rolled up into a spiral form with a striplike separator 52interposed between the electrodes. The positive electrode 51 is preparedby coating opposite surfaces of a striplike current collector 55 ofaluminum foil with a positive electrode active material 54 comprising alithium containing composite oxides. The negative electrode 53 isprepared by coating opposite surfaces of a striplike current collector57 of copper foil with a negative electrode active material 56containing a carbon material. The separator 52 is impregnated with anonaqueous electrolyte.

[0014] The positive electrode 51 has an uncoated portion having noactive material 54 applied thereto, and base ends of the electrode tabs6 are joined to the uncoated portion. Similarly, the negative electrode53 has an uncoated portion having no active material 56 applied thereto,and base ends of the electrode tabs 6 are joined to the uncoatedportion.

[0015] With reference to FIG. 23, the electrode tabs 6 of the samepolarity have outer ends 61 connected to one electrode terminal assembly110. For the sake of convenience, FIG. 23 shows only some of theelectrode tabs as connected at their outer ends to the terminal assembly110, with the connection of the other tab outer ends to the assembly 110omitted from the illustration.

[0016] The electrode terminal assembly 110 comprises an electrodeterminal 111 extending through and attached to the lid 16 of the batterycan 1. The electrode terminal 111 has a base end formed with a flange112. The hole in the lid 16 for the terminal 111 to extend therethroughhas an insulating packing 113 fitted therein to provide electricalinsulation and a seal between the lid 16 and fastening members. Theterminal 111 has a washer 114 fitted therearound from outside the lid16, and a first nut 115 and a second nut 116 which are screwed thereon.The insulating packing 113 is clamped between the flange 112 of theterminal 111 and the washer 114 by tightening up the first nut 115 toproduce an enhanced sealing effect. The outer ends 61 of the electrodetabs 6 are secured to the flange 112 of the terminal 111 by spot weldingor ultrasonic welding.

[0017] Lithium ion secondary cells have the problem that an increase inthe size thereof lengthens the positive and negative electrodes,consequently lowering the current collecting efficiency of the currentcollecting structure comprising electrode tabs to produce variations ininternal resistance or result in a lower discharge capacity.

[0018]FIG. 21 shows a current collecting structure proposed to obtain auniform current collecting efficiency over the entire lengths of thepositive and negative electrodes. The proposed structure is provided fora rolled-up electrode unit 7, which comprises a positive electrode 71prepared by coating a current collector 75 with a positive electrodeactive material 74, a negative electrode 73 formed by coating a currentcollector 77 with a negative electrode active material 76 and aseparator 72 impregnated with a nonaqueous electrolyte. The positiveelectrode 71 and the negative electrode 73 are each superposed on theseparator 72 as displaced widthwise of the separator, and rolled up intoa spiral form, whereby the edge 78 of current collector 75 of thepositive electrode 71 is positioned as projected outward beyond the edgeof the separator 72 at one of opposite ends of the electrode unit 7 inthe direction of its winding axis, and the edge 78 of current collector77 of the negative electrode 73 is positioned as projected outwardbeyond the edge of the separator 72 at the other end of the unit 7.

[0019] A disklike current collector plate 62 is secured to each ofopposite ends of the rolled-up electrode unit 7 by resistance weldingand connected to the same electrode terminal assembly 110 as describedabove by a lead member 63.

[0020] The nonaqueous electrolyte secondary cell with the currentcollecting structure of FIG. 21, however, has the problem of being greatin the internal resistance of the cell because the edges 78, 78 of thecurrent collectors 75, 77 forming the positive electrode 71 and thenegative electrode 73 of the electrode unit 7 have a small area,therefore providing a small area of contact between the collector plate62 and each current collector edge.

[0021] It is especially required that lithium ion secondary cells, forexample, for use as power sources in electric motor vehicles be of highcapacity and reduced in internal resistance to the greatest possibleextent so as to obtain a high power. Furthermore a current collectingstructure of high productivity is required for a reduction ofmanufacturing cost.

[0022] Accordingly, a cell of low resistance and high productivity hasbeen proposed which comprises a current collector plate having smallbulging portions formed thereon as uniformly distributed over the entiresurface thereof, such that the collector plate is secured to a currentcollector edge by resistance welding with the bulging portions incontact therewith to concentrate the current on the bulging portions andgive improved weld strength (see, for example, JP-U No. 156365/1980).

[0023] As shown in FIG. 24, also proposed is a current collectingstructure which comprises a current collector plate 92 prepared byforming a plurality of bent portions 94 on a flat platelike body 93, thebent portions 94 being secured to a current collector edge 78 of arolled-up electrode unit 7 by resistance welding with the collectorplate 92 pressed against the current collector edge 78 (see, forexample, JP-A No. 31497/1999).

[0024] Further known are a current collector plate comprising twodivided segments for suppressing ineffective current involved inattaching the collector plate by resistance welding to achieve animproved welding efficiency (JP-A No. 29564/1995), and a currentcollector plate having a projection V-shaped in section and formed onthe portion thereof to be joined by resistance welding so as toconcentrate the welding current on the projection and afford improvedweld strength (JP-B No. 8417/1990).

[0025] Further proposed is a current collecting structure comprising acurrent collector member 95 in place of the disklike collector plate andformed with a plurality of slits 96 as seen in FIG. 25. For laserwelding, a laser beam is projected onto the surface of the collectormember 95 as disposed at an end of a rolled-up electrode unit 7, with acurrent collector edge 78 fitted in the slits 96 of the member 95 (JP-ANo. 261441/1998).

[0026] Also proposed is a structure wherein a disklike current collectorplate has a plurality of projections, V-shaped in section and up to 90°in end angle, and is welded to a group of electrode plates byirradiating the projections with a laser beam, with the collector platepressed against each current collector (JP-B No. 4102/1990).

[0027] However, with the above-mentioned current collecting structurewherein the current collector plate is formed with small bulgingportions as uniformly distributed over the entire surface thereof (JP-UNo. 156365/1980), the collector plate is in unstable contact with thecurrent collector, and the current fails to flow across these membersdepending on the state of contact, entailing the problem of producing afaulty weld.

[0028] The current collecting structure wherein the current collectorplate has projections which are V-shaped in section or bent portions forthe resistance welding of the plate (JP-A No. 31497/1999, No. 29564/1995or JP-B No. 8417/1990) has the problem of low weld strength when thecurrent collector has a very small thickness as is the case with lithiumion secondary cells.

[0029] The current collecting structure wherein the current collectormember having a plurality of slits is secured to the current collectoredge by laser welding (JP-A No. 261441/1998) not only requires thecollector member which has a complex shape but also has the problem thatthe work of inserting the current collector edge into the slits of thecollector member is very cumbersome.

[0030] With the structure wherein the disklike current collector platehaving projections of V-shaped section is joined to the group ofelectrode plates by laser welding (JP-B No. 4102/1990), the projectionshave a V-shaped section of acute angle, so that the area of contactbetween the projection and the current collector edge is small,consequently entailing the problem of increased contact resistance.Since the junction between the V-shaped projection and the currentcollector edge is at an acute angle with the direction of projection ofthe laser beam for irradiating the junction, the laser beam fails to acteffectively to weld the junction and is likely to produce a faulty weld.

SUMMARY OF THE INVENTION

[0031] A first object of the present invention is to provide theconstruction of a nonaqueous electrolyte secondary cell having a currentcollecting structure wherein a negative electrode current collectorplate is secured to an end of an electrode unit by welding, and toprovide a process for fabricating the cell, the collector plate havingimproved weldability to the electrode unit.

[0032] A second object of the invention is to provide a nonaqueouselectrolyte secondary cell having a current collecting structure whichis high in productivity and which is so adapted that even when a currentcollector forming an electrode unit is very thin, an edge of the currentcollector can be joined to a current collector plate over an increasedarea of contact, and a process for fabricating the cell.

[0033] Construction for Fulfilling First Object

[0034] The present invention provides a nonaqueous electrolyte secondarycell comprising an electrode unit 2 which includes a negative electrode21 having a projecting edge at one of opposite ends of the electrodeunit in the direction of winding axis thereof. A negative electrodecurrent collector plate 3 is joined to the edge and electricallyconnected to a negative terminal portion. The collector plate 3comprises a plurality of layers including a copper layer 31 made ofcopper or an alloy consisting predominantly of copper, and a metal layermade of a metal not forming an intermetallic compound with lithium andhaving a lower laser beam reflectivity than copper or an alloyconsisting predominantly of the metal. The copper layer 31 and the metallayer provide opposite surface layers of the collector plate 3, and thecopper layer 31 is welded to the edge of the negative electrode 21. Themetal for forming the metal layer of the negative electrode currentcollector plate 3 is, for example, nickel, stainless steel, titanium,chromium or molybdenum.

[0035] When the collector plate 3 is welded to the negative electrodeedge of the electrode unit 2 with a laser beam in the process forfabricating the nonaqueous electrolyte secondary cell of the invention,the laser beam can be sufficiently absorbed by the collector plate 3 forperfect welding since the laser beam impinging side of the plate 3 isprovided by the metal layer which is low in laser beam reflectivity.

[0036] The metal layer of the collector plate 3 is made of a metal notforming an intermetallic compound with lithium or an alloy consistingpredominantly of the metal and is therefore unlikely to consume lithiumions in the nonaqueous electrolyte to form an alloy, consequentlyprecluding the lithium ion concentration of the nonaqueous electrolytefrom reducing.

[0037] Further because the negative electrode current collector plate 3comprises a plurality of layers, i.e., the copper layer 31 and the metallayer, the high conductivity of the copper layer gives the plate 3 lowerelectric resistance and higher electric conductivity than when the plate3 consists solely of the metal layer.

[0038] The edge of the negative electrode 21 of the electrode unit 2 isjoined to the copper layer 31 of the collector plate 3 over the entirelength thereof, consequently making it possible to collect the currentfrom the entire electrode unit 2 uniformly even if the cell islarge-sized with an increase in the length of the electrodes. Thisreduces the potential gradient along the length of the negativeelectrode 21, giving a uniform current distribution, whereby a highcurrent collecting efficiency can be achieved.

[0039] Stated more specifically, the negative electrode currentcollector plate 3 has a thickness in the range of 0.10 mm to 5.00 mm. Ifthe thickness is smaller than 0.10 mm, the collector plate 3 itself hasincreased electric resistance, which not only results in a lower currentcollecting efficiency but also permits the collector plate 3 to becomemelted to excess by laser welding to produce a cave-in in the weld. Ifthe thickness is in excess of 5.00 mm, on the other hand, welding of thecollector plate 3 requires increased power, presenting difficulty inwelding the collector plate 3 to the negative electrode edge which istens of micrometers in thickness.

[0040] Further stated more specifically, the ratio of the thickness ofthe metal layer to the thickness of the negative electrode currentcollector plate 3 is in the range of at least 5% to not greater than45%. This enables the metal layer to fully serve the function ofexhibiting reduced laser beam reflectivity, also permitting the copperlayer 31 to satifactorily perform the function of exhibiting reducedelectric resistance. If the ratio is smaller than 5%, the metal layerdisappears on melting immediately after the start of welding of thecollector plate 3 to expose a surface of high laser beam reflectivity,hence impaired weldability. When the ratio is in excess of 45%, on theother hand, the metal layer becomes predominant with respect to theelectric resistance of the collector plate 3, increasing the overallelectric resistance of the plate 3.

[0041] The present invention further provides a process for fabricatinga nonaqueous electrolyte secondary cell which process has the steps of:

[0042] preparing an electrode unit 2 by laying a positive electrode 23and a negative electrode 21 over each other with a separator 22sandwiched therebetween so as to project an edge of the positiveelectrode 23 at one of opposite ends of the electrode unit 2 and toproject an edge of the negative electrode 21 at the other end androlling up the resulting assembly into a spiral form,

[0043] preparing a positive electrode current collector plate 30 fromaluminum or an alloy consisting predominantly of aluminum,

[0044] preparing a negative electrode current collector plate 3comprising a plurality of layers including a copper layer 31 made ofcopper or an alloy consisting predominantly of copper, and a metal layermade of a metal not forming an intermetallic compound with lithium andhaving a lower laser beam reflectivity than copper or an alloyconsisting predominantly of the metal, the copper layer 31 and the metallayer providing respective opposite surface layers of the collectorplate 3,

[0045] welding the positive electrode current collector plate 30 to theedge of the positive electrode 23 by placing the collector plate 30 atthe end of the electrode unit 2 having the projecting edge of thepositive electrode 23 and irradiating a surface of the collector plate30 with a laser beam,

[0046] welding the negative electrode current collector plate 3 to theedge of the negative electrode 21 by placing the collector plate 3 atthe end of the electrode unit 2 having the projecting edge of thenegative electrode 21, with the copper layer 31 in contact with thenegative electrode edge, and irradiating a surface of the metal layer ofthe collector plate 3 with a laser beam, and

[0047] assembling a nonaqueous electrolyte secondary cell byelectrically connecting the positive electrode current collector plate30 and the negative electrode current collector plate 3 which are weldedto the electrode unit 2 to a positive terminal portion and a negativeterminal portion respectively.

[0048] In the step of welding the negative electrode current collectorplate 3 to the edge of the negative electrode 21 with a laser beam inthe fabrication process of the invention described above, the laser beamis projected on the surface of the metal layer of low reflectivity, sothat the energy of the laser beam can be fully given to the junction ofthe collector plate 3 and the edge of the negative electrode 21,consequently welding the plate 3 and the negative electrode edge to eachother completely.

[0049] In the step of welding the positive electrode current collectorplate 30 to the edge of the positive electrode 23 with a laser beam, thealuminum forming the collecting plate 30 is low in laser beamreflectivity, so that the energy of the laser beam can be fully given tothe junction of the collector plate 30 and the edge of the positiveelectrode 23, consequently welding the plate 30 and the positiveelectrode edge to each other completely.

[0050] In the assembling step, the positive electrode current collectorplate 30 and the negative electrode current collector plate 3 areelectrically connected to the positive terminal portion and the negativeterminal portion, respectively.

[0051] This sufficiently lowers the electric resistance of theconductors extending from the electrode unit 2 to the terminal portionsto achieve a high current collecting efficiency.

[0052] The nonaqueous electrolyte secondary cell and the process forfabricating the cell according to the invention give the negativeelectrode current collector plate improved weldability to the electrodeunit, whereby a high current collecting efficiency can be attained asdescribed above.

[0053] Construction for Fulfilling Second Object

[0054] Another nonaqueous electrolyte secondary cell comprises anelectrode unit 7 encased in a battery can 1 and comprising as superposedin layers a positive electrode 71, a negative electrode 73 and aseparator 72 interposed between the electrodes and impregnated with anonaqueous electrolyte, each of the positive electrode 71 and thenegative electrode 73 being formed by coating a striplike currentcollector with an active material. The cell is adapted to deliverelectric power generated by the electrode unit 7 to the outside via apair of electrode terminals.

[0055] The current collector of the positive electrode 71 or thenegative electrode 73 has a projecting edge 78 at at least one ofopposite ends of the electrode unit 7, and a current collector plate 8is joined to the edge 78 and has a plurality of protrusions 82 formed ona surface thereof opposed to the current collector edge 78. Each of theprotrusions is shaped to have a circular-arc section or polygonal (e.g.,trapezoidal) section with at least four corners, the collector plate 8being welded to the current collector edge 78 with the protrusions 82forced therein and being connected to one of the electrode terminals.

[0056] The present invention further provides a process for fabricatinga nonaqueous electrolyte secondary cell which process has the steps of:

[0057] preparing an electrode unit 7 wherein an edge 78 of currentcollector of each of a positive electrode 71 and a negative electrode 73is positioned as projected outward beyond an edge of a separator 72 bylaying the positive electrode 71 and the negative electrode 73 over theseparator 72 as displaced from the separator widthwise thereof androlling up the resulting assembly into a spiral form,

[0058] preparing current collector plates 8 each by forming in a flatplatelike body 81 having electric conductivity a plurality ofprotrusions 82 each shaped to have a circular-arc section or polygonalsection having at least four corners, welding the collector plates 8respectively to the projecting current collector edges 78 at therespective ends of the electrode unit 7 by placing each collector plate8 over the current collector edge 78 in pressing contact therewith andirradiating each protrusion 82 of the collector plate 8 with a laserbeam or electron beam, with the protrusion 82 forced into the currentcollector edge 78, and

[0059] placing the electrode unit 7 having the collector plates 8 weldedthereto into a battery can 1 and connecting the collector plates 8 torespective electrode terminals.

[0060] With the nonaqueous electrolyte secondary cell and thefabrication process thereof according to the invention described, thecurrent collector plate 8 is pressed against the current collector edge78 of the electrode unit 7, whereby each protrusion 82 of the collectorplate 8 is forced or wedged into the current collector edge 78, forminga joint face in the current collector edge 78 in conformity with thecontour of the protrusion 82, for example, a cylindrical joint face. Thejoint face has a larger area than is formed by a protrusion which isV-shaped in section.

[0061] Accordingly, when the collector plate 8 is welded to the currentcollector edge 78 by irradiating the junction of each protrusion 82 andthe current collector edge 78 with a laser beam or electron beam, theplate 8 is joined to the current collector edge 78 over a large area ofcontact. This results in diminished contact resistance and a highercurrent collecting efficiency.

[0062] The junction of the collector plate protrusion 82 and the currentcollector edge 78 will be positioned at 90° or approximately at thisangle with the direction of projection of the beam at the midportion ofthe junction, so that the laser beam or electron beam acts effectivelyfor welding the junction, consequently affording a high weld strengthdue to the large area of the junction.

[0063] Stated more specifically, the current collector plate 8 comprisesa flat platelike body 81 formed with the protrusions 82 and one or aplurality of liquid inlets 83, and the opening area provided by theliquid inlets 83 is at least 15% of the flat area of the body. When theelectrolyte is placed into the cell 1 can in the step of assembling thecell, the electrolyte flows through the liquid inlets 83 in the currentcollector plate 8 of this structure and is fed to the electrode unit 7.This shortens the time required to impregnate the separator 72, positiveelectrode 71 and negative electrode 73 with the electrolyte. If theopening ratio provided by the liquid inlets 83 is smaller than 15%, theelectrolyte encounters difficulty in passing through the collector plate8 and therefore requires a prolonged period of time for impregnation.However, if the opening ratio given by the liquid inlets 83 is in excessof 90%, the current path becomes greatly constricted, increasing theelectric resistance of the collector plate 8 and leading to a lowercurrent collecting efficiency. Accordingly, it is desirable that theopening ratio given by the liquid inlets 83 be in the range of 15% to90%.

[0064] Alternatively, the current collector plate 8 comprises a flatplatelike body 81 formed with the protrusions 82 and integrally providedwith a striplike lead portion 85, the lead portion 85 having an outerend connected to the electrode terminal. The lead portion 85 of thisstructure is easily connectable to the electrode terminal, furtherserving to diminish the electric resistance between the electrode unit 7and the electrode terminal.

[0065] A current collector plate 100 of another structure comprises aflat platelike body 101 provided at an outer peripheral portion thereofwith a current collector pressing portion 106 positioned in the vicinityof each protrusion 102 for pressing an end portion of the currentcollector 77 of the electrode unit 7 inwardly of the electrode unit 7.With this structure, the end portion of the current collector 77 isdeflected inwardly of the electrode unit 7 by being pressed by thecurrent collector pressing portion 106, whereby the position of contactof the current collector end with the protrusion 102 of the collectorplate 100 is shifted also inwardly of the electrode unit 7. Accordingly,when the collector plate protrusion 102 is to be welded to the endportion of the current collector 77, the laser beam or electron beamneed not be projected onto the radial outer end of the protrusion butthe protrusion needs only to be irradiated up to a position slightlyinwardly of its outer end, i.e., up to the position where the deflectedportion of the current collector 77 is in contact with the protrusion.This eliminates the likelihood that the beam will be projected outsidebeyond the outer periphery of the collector plate 100, consequentlyprecluding the current collector 77 or separator 72 from melting bybeing directly irradiated with the beam.

[0066] The pressing face of the current collector pressing portion 106for the current collector 77 and the surface of the platelike body 101of the collector plate 100 make an angle in the range of at least 30° tonot greater than 45°. When the angle is limited to this range, the outerend of the current collector 77 can be effectively deflected inwardly ofthe electrode unit 7.

[0067] According to the process of the invention described forfabricating nonaqueous electrolyte secondary cells, it is desirable thatthe protrusions 82 of the collector plate 8 have a width at least 0.8times the diameter of the spot of the laser beam or electron beam. Forexample, when the protrusion 82 of the collector plate 8 has asemicircular form in section, it is desired that the diameter of thesemicircle be at least 0.8 times the spot diameter of the laser beam orelectron beam. Further when the collector plate protrusion 82 has atrapezoidal form in section, it is desired that the width of the upperside (short side) of the trapezoid be at least 0.8 times the spotdiameter of the laser beam or electron beam. This enables the laser beamor electron beam to give energy concentrically on the junction of thecollector plate protrusion 82 and the current collector edge 78, fullymelting the portions to be joined and giving a large joint area and highweld strength.

[0068] The distance the protrusion 82 of the collector plate 8 projectsis preferably at least 0.5 mm to not greater than 3 mm. If the distanceof projection of the protrusion 82 is smaller than 0.5 mm, it isimpossible to force the protrusion 82 into all turns of the currentcollector at the edge 78 in the case where the edge portions 78 of turnsof the current collector of the electrode unit 7 are not positioneduniformly in a plane, consequently failing to afford sufficient weldstrength. Further when the distance of projection of the protrusion 82is in excess of 3 mm, the effect to improve the weld strength will leveloff, while a greater dead space is created in the interior of thebattery can 1 to entail a lower energy density relative to the volume.

[0069] The thickness of the current collector plate 8 is preferably atleast 0.1 mm to not greater than 2 mm. If the thickness is smaller than0.1 mm, the collector plate 8 has increased electric resistance toexhibit a lower current collecting efficiency. Further if the thicknessis greater than 2 mm, the effect to improve the current collectingefficiency levels off, while the lead portion 85 formed integrally withthe plate 8 will not be workable without a problem.

[0070] Further it is desired that the wall thickness of the protrusion82 of the current collector plate 8 be smaller than the thickness of theflat platelike body 81. The flat portion then has a greater thickness,ensuring a satisfactory current collecting efficiency withoutimpairment, while the portion to be irradiated with a beam has a smallthickness and therefore permits welding with low energy.

[0071] Usable as the material for the current collector plate 8 is Cu,Al, Ni, SUS, Ti or an alloy of such metals. Use of these materialsprovides cells which are excellent in corrosion resistance to nonaqueouselectrolytes and in conductivity.

[0072] According to the present invention providing nonaqueouselectrolyte secondary cells and processes for fabricating such cells,the current collector plate can be joined to the current collector edgeover a large contact area even if the current collector forming theelectrode unit has a very small thickness as described above, hence highproductivity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0073]FIG. 1 is a view in section of a cylindrical lithium ion secondarycell according to the invention;

[0074]FIG. 2 is a perspective view of a negative electrode currentcollector plate;

[0075]FIG. 3 is a sectional view showing the step of welding thenegative electrode current collector plate to a rolled-up electrode unitwith a laser beam;

[0076]FIG. 4 is a perspective view partly in development of therolled-up electrode unit;

[0077]FIG. 5 is a perspective view of a negative electrode currentcollector plate of another structure;

[0078]FIG. 6 is a perspective view of a negative current collector plateof still another structure;

[0079]FIG. 7 is a view in section of a conventional cylindrical lithiumion secondary cell;

[0080]FIG. 8 is a fragmentary front view partly broken away and showinga lithium ion secondary cell embodying the invention;

[0081]FIG. 9 is an exploded perspective view of a rolled-up electrodeunit and a current collector plate;

[0082]FIG. 10 is a plane view of the collector plate;

[0083]FIG. 11 is an enlarged view in section taken along the line A-A inFIG. 10;

[0084]FIG. 12 is a perspective view showing the step of pressing thecollector plate against the rolled-up electrode unit;

[0085]FIG. 13 is a sectional view showing a circular-arc protrusion ofthe collector plate as forced into a current collector edge;

[0086]FIG. 14 is a sectional view showing a V-shaped protrusion of acurrent collector plate as forced into a current collector edge;

[0087]FIG. 15 is a sectional view showing a trapezoidal protrusion of acurrent collector plate as forced into a current collector edge;

[0088]FIG. 16 is a perspective view of a negative electrode currentcollector plate of another structure;

[0089]FIG. 17 is a plane view of the collector plate;

[0090]FIG. 18 is a plane view for illustrating the position of a laserbeam spot on the collector plate;

[0091]FIG. 19 is a view in section taken along the line E-E in FIG. 18;

[0092]FIG. 20 is a perspective view showing the appearance of anotherconventional cylindrical lithium ion secondary cell;

[0093]FIG. 21 is an exploded perspective view of a current collectorplate and a rolled-up electrode unit;

[0094]FIG. 22 is a perspective view partly in development and showingthe rolled-up electrode unit used in the conventional lithium ionsecondary cell;

[0095]FIG. 23 is a fragmentary front view partly broken away and showingthe conventional cell;

[0096]FIG. 24 is an exploded perspective view showing a currentcollector plate and a rolled-up electrode unit of the prior art; and

[0097]FIG. 25 is an exploded perspective view showing other currentcollector plate and rolled-up electrode unit of the prior art.

DETAILED DESCRIPTION OF EMBODIMENTS

[0098] Cylindrical lithium ion secondary cells embodying the presentinvention will be described below with reference to the drawings.

[1] First Embodiment

[0099] As shown in FIG. 1, the cylindrical lithium ion secondary cell ofthis embodiment comprises a battery can 1 formed by fixing lids 16, 16to opposite open ends of a cylinder 15. A rolled-up electrode unit 2 isencased in the battery can 1. Arranged respectively at opposite ends ofthe electrode unit 2 are a negative electrode current collector plate 3and a positive electrode current collector plate 30 each comprising twolayers, i.e., a copper layer 31 and a metal layer made of a metal notforming an intermetallic compound with lithium and having a lower laserbeam reflectivity than copper or an alloy consisting predominantly ofthe metal. Each collector plate is welded to the end of the unit 2 witha laser beam. The collector plates 3 and 30 are connected by respectivelead portions 33, 34 to a negative terminal assembly 4 and a positiveterminal assembly 40 mounted on the lids 16, 16.

[0100] With reference to FIG. 4, the rolled-up electrode unit 2comprises a positive electrode 23, separator 22 and negative electrode21 each in the form of a strip. The positive electrode 23 is formed bycoating a current collector of aluminum foil with a positive electrodeactive material 26 comprising LiCoO₂. The negative electrode 21 isformed by coating a current collector of copper foil with a negativeelectrode active material 24 comprising natural graphite.

[0101] The positive electrode 23 and the negative electrode 21 are eachsuperposed on the separator 22 as displaced widthwise thereof and arerolled up into a spiral form, whereby an edge (uncoated portion 25) ofthe rolled-up negative electrode 21 is positioned as projected outwardbeyond the edge of the separator 22 at one of opposite ends of theelectrode unit 2 in the direction of its winding axis, and an edge(uncoated portion 27) of the rolled-up positive electrode 23 ispositioned as projected outward beyond the edge of the separator 22 atthe other end of the unit 2.

[0102] For example, the active material coatings 24, 26 of theelectrodes can be tens of millimeters in width A, the uncoated portions25, 27 about 10 mm in width B, and the distance S of projection beyondthe separator 22 about 1 to about 3 mm.

[0103] As shown in FIGS. 1 and 2, the negative electrode currentcollector plate 3 is in the form of a disk and has a two-layerstructure, i.e., a copper layer 31 having a thickness of 2.40 mm and anickel layer 32 having a thickness of 0.60 mm and made from nickel whichis a metal not forming an intermetallic compound with lithium and havinga lower laser beam reflectivity than copper. The lead portion 33, whichis made of copper, extends from an end portion of the collector plate 3.Also usable as the collector plate 3 is one having the same structure asabove except that the nickel layer 32 is replaced by a stainless steellayer 35 as seen in FIG. 5. Further usable is a negative electrodecurrent collector plate 3 having a three-layer structure, i.e., a copperlayer 31 and a nickel layer 32 providing opposite surface layers and astainless steel layer 39 sandwiched between the two layers. Furthermore,the nickel layer 32 or stainless steel layer 35 can be replaced by alayer of a metal, such as titanium layer, chromium layer or molybdenumlayer, insofar as the metal forms no intermetallic compound with lithiumand has a lower laser beam reflectivity than copper.

[0104] As shown in FIG. 1, on the other hand, the positive currentcollector plate 30 is similarly in the form of a disk, made from analuminum plate having a thickness of 1.00 mm and provided with the leadportion 34 which is made of aluminum.

[0105] With reference to FIG. 3, the negative electrode currentcollector plate 3 is disposed at one end of the electrode unit 2 withthe copper layer 31 in contact with the edge (uncoated portion 25) ofthe negative electrode 21 of the unit 2, and is welded to the edge ofthe negative electrode 21 by being irradiated with a laser beam over thesurface of the nickel layer 32.

[0106] The positive electrode current collector plate 30 is disposedlikewise at the other end of the electrode unit 2 and welded to the edgeof the positive electrode 23 by being irradiated with a laser beam overthe surface thereof.

[0107] As shown in FIG. 1, the negative terminal assembly 4 comprises aterminal member 41 having a screw shank 42 and a flange 43 projectingfrom the lower end of the shank 42. The screw shank 42 of the terminalmember 41 extends through the lid 16, and a first insulating member 45and a second insulating member 46 are fitted around the terminal member41 to provide electrical insulation and a seal between the lid 16 andthe terminal member 41. The terminal member 41 has a washer 47 fittedtherearound and a nut 48 screwed on its outer end. The positive terminalassembly 40 also has the same construction as the assembly 4.

[0108] The lead portion 33 extending from the collector plate 3 has itsouter end welded to the flange 43 of terminal member 41 of the negativeterminal assembly 4. The lead portion 34 extending from the positivecollector plate 30 has its outer end welded to the flange 43 of terminalmember 41 of the positive terminal assembly 40. This arrangement makesit possible to deliver the power generated by the rolled-up electrodeunit 2 from the negative and positive terminal assemblies 4, 40.

[0109] The lithium ion secondary cell of the present invention isfabricated by the process to be described below.

[0110] Preparation of Rolled-up Electrode Unit 2

[0111] A positive electrode 23 is prepared by mixing together a positiveelectrode active material comprising LiCoO₂, an auxiliary conductiveagent comprising carbon and a binder comprising polyvinylidene fluoride(PVdF) to obtain a positive electrode composition and coating oppositesurfaces of a current collector in the form of a strip of aluminum foilwith the composition as shown in FIG. 4. The positive electrode currentcollector has one edge portion left uncoated with the active materiallayer to provide an uncoated portion 27 of 10 mm in width.

[0112] A negative electrode 21 is prepared by mixing together a negativeelectrode active material comprising natural graphite and a bindercomprising polyvinylidene fluoride (PVdF) to obtain a negative electrodecomposition and coating opposite surfaces of a current collector in theform of a strip of copper foil with the composition. The negativeelectrode current collector has one edge portion left uncoated with theactive material layer to provide an uncoated portion 25 of 10 mm inwidth.

[0113] Further prepared is a separator 22 having a width slightly largerthan the width A of the coated portion of the positive electrode and thecoated portion of the negative electrode. The separator 22 is made fromporous polyethylene and polypropylene.

[0114] The positive electrode 23, separator 22 and negative electrode 21are thereafter laid over one another and rolled up into a spiral form asshown in FIG. 4 to obtain a rolled-up electrode unit 2. At this time,these components are arranged in layers so that the edges of thepositive electrode uncoated portion 27 and the negative electrodeuncoated portion 25 are positioned as projected outward beyond therespective edges of the separator 22.

[0115] Preparation of Current Collector Plates 30, 3

[0116] A negative electrode current collector plate 3 of two-layerstructure is prepared which comprises a copper layer 31 with a thicknessof 2.40 mm and a nickel layer 32 with a thickness of 0.60 mm as shown inFIG. 2. Alternatively prepared is a negative electrode current collectorplate 3 of two-layer structure comprising a stainless steel layer 35 asseen in FIG. 5, or a negative electrode current collector plate 3 ofthree-layer structure comprising a copper layer 31 with a thickness of2.40 mm, a nickel layer 32 with a thickness of 0.30 mm and a stainlesssteel layer 39 having a thickness of 0.30 mm and sandwiched betweenthese layers 31, 32 as seen in FIG. 6. A lead portion 33 of copper isjoined at the base end thereof to an end portion of the collector plate3. Further prepared is a positive electrode current collector plate 30comprising an aluminum sheet with a thickness of 1.00 mm. A lead portion34 of aluminum is joined at its base end to an end portion of thecollector plate 30.

[0117] Assembly of Cell

[0118] The negative electrode current collector plate 3 is welded to theedge of the negative electrode 21 by positioning the collector plate 3at one end of the electrode unit 2 with the copper layer 31 in contactwith the edge of the negative electrode 21 of the unit 2 and irradiatingthe surface of the nickel layer 32 of the plate 3 with a laser beam. Thepositive electrode current collector plate 30 is welded to the edge ofthe positive electrode 23 of the electrode unit 2 by positioning thecollector plate 30 at the edge of the positive electrode 23 andirradiating the surface of the collector plate 30 with a laser beam.

[0119] Subsequently, the outer end of the lead portion 33 extending fromthe collector plate 3 is joined to the flange 43 of terminal member 41of a negative terminal assembly 4 by ultrasonic welding, and the outerend of the lead portion 34 extending from the positive collector plate30 is joined to the flange 43 of terminal member 41 of a positiveterminal assembly 4 by ultrasonic welding. The negative terminalassembly 4 and the positive terminal assembly 40 are mounted onrespective lids 16, 16.

[0120] The rolled-up electrode unit 2 is inserted into a cylinder 15,the lids 16, 16 are welded to the open ends of the cylinder 15, and anelectrolyte is thereafter poured into the cylinder through anunillustrated electrolyte inlet. The electrolyte is prepared by mixingethylene carbonate and diethyl carbonate together in a volume ratio of1:1 and dissolving LiPF₆ in the solvent mixture at a concentration of 1mole/liter. The electrolyte inlet is eventually sealed off. In this way,a cylindrical lithium ion secondary cell is completed as shown in FIG.1.

[0121] The positive electrode active material is not limited to LiCoO₂mentioned above; also usable are LiNiO₂, LiMn₂O₄, etc. The negativeelectrode active material is not limited to natural graphite mentionedabove; also usable are other carbon materials such as artificialgraphite, coke, etc. and materials capable of absorbing and desorbinglithium. The electrolyte is not limited to the above-mentioned one; alsousable are solutions having a concentration of 0.7 to 1.5 moles/literand prepared by dissolving a solute, such as LiClO₄ or LiCF₃SO₄, in asolvent mixture of vinylidene carbonate, propylene carbonate or likeorganic solvent and dimethyl carbonate, diethyl carbonate,1,2-dimethoxyethane, ethoxymethoxyethane or like low-boiling pointsolvent.

[0122] Experiment

[0123] Invention cells 1 to 11 were fabricated which had the sameconstruction as the cylindrical lithium ion secondary cell of theinvention described above. These cells each had a negative electrodecurrent collector plate 3 of two-layer structure as shown in FIG. 2 andmade different in the thicknesses of the nickel layer 32 and copperlayer 31. Also fabricated were invention cells 12 to 22 each of whichhad a negative electrode current collector plate 3 of two-layerstructure as shown in FIG. 5 and which were made different in thethicknesses of the stainless steel layer 35 and copper layer 31. Furtherfabricated was invention cell 23 which had a negative electrode currentcollector plate 3 comprising three layers, i.e., a nickel layer 32,stainless steel layer 39 and copper layer 31 as shown in FIG. 6. On theother hand, comparative cells 1 and 2 were fabricated which had the sameconstruction as the invention cells except that the negative electrodecurrent collector plate had a single-layer structure comprising a nickelor copper plate as seen in FIG. 7. The cells were checked for powerdensity. The stainless steel used was an austenitic stainless steel.

[0124] Tables 1 to 6 show the constructions of the cells. TABLE 1 RATIOOF THICKNESS THICKNESS OF Ni LAYER OF NEGATIVE TO THICKNESS THICKNESSTHICKNESS COLLECTOR OF COLLECTOR OF Ni LAYER OF Cu LAYER PLATE PLATECELL NO. (mm) (mm) (mm) (%) INVENTION 0.02 0.07 0.09 22 CELL 1 INVENTION0.02 0.08 0.10 20 CELL 2 INVENTION 0.20 0.80 1.00 20 CELL 3 INVENTION0.60 2.40 3.00 20 CELL 4 INVENTION 1.00 4.00 5.00 20 CELL 5 INVENTION1.10 4.40 5.50 20 CELL 6

[0125] TABLE 2 RATIO OF THICKNESS THICKNESS OF Ni LAYER OF NEGATIVE TOTHICKNESS THICKNESS THICKNESS COLLECTOR OF COLLECTOR OF Ni LAYER OF CuLAYER PLATE PLATE CELL NO. (mm) (mm) (mm) (%) INVENTION 0.12 2.88 3.00 4CELL 7 INVENTION 0.15 2.85 3.00 5 CELL 8 INVENTION 0.30 2.70 3.00 10CELL 9 INVENTION 0.60 2.40 3.00 20 CELL 4 INVENTION 1.35 1.65 3.00 45CELL 10 INVENTION 1.40 1.60 3.00 47 CELL 11

[0126] TABLE 3 RATIO OF THICKNESS OF STAINLESS THICKNESS STEEL LAYERTHICKNESS OF OF NEGATIVE TO THICKNESS STAINLESS THICKNESS COLLECTOR OFCOLLECTOR STEEL LAYER OF Cu LAYER PLATE PLATE CELL NO. (mm) (mm) (mm)(%) INVENTION 0.02 0.07 0.09 22 CELL 12 INVENTION 0.02 0.08 0.10 20 CELL13 INVENTION 0.20 0.80 1.00 20 CELL 14 INVENTION 0.60 2.40 3.00 20 CELL15 INVENTION 1.00 4.00 5.00 20 CELL 16 INVENTION 1.10 4.40 5.50 20 CELL17

[0127] TABLE 4 RATIO OF THICKNESS OF STAINLESS THICKNESS STEEL LAYERTHICKNESS OF OF NEGATIVE TO THICKNESS STAINLESS THICKNESS COLLECTOR OFCOLLECTOR STEEL LAYER OF Cu LAYER PLATE PLATE CELL NO. (mm) (mm) (mm)(%) INVENTION 0.12 2.88 3.00 4 CELL 18 INVENTION 0.15 2.85 3.00 5 CELL19 INVENTION 0.30 2.70 3.00 10 CELL 20 INVENTION 0.60 2.40 3.00 20 CELL15 INVENTION 1.35 1.65 3.00 45 CELL 21 INVENTION 1.40 1.60 3.00 47 CELL22

[0128] TABLE 5 RATIO OF THICKNESS OF STAINLESS STEEL LAYER + NiTHICKNESS LAYER TO THICKNESS THICKNESS OF NEGATIVE THICKNESS OF OF Ni OFSTAINLESS THICKNESS COLLECTOR COLLECTOR LAYER STEEL LAYER OF Cu LAYERPLATE PLATE CELL NO. (mm) (mm) (mm) (mm) (%) INVENTION 0.30 0.30 2.403.00 20 CELL 23

[0129] TABLE 6 THICKNESS OF NEGATIVE THICKNESS OF Ni THICKNESS COLLECTORLAYER OF Cu LAYER PLATE CELL NO. (mm) (mm) (mm) COMP. 0.00 3.00 3.00CELL 1 COMP. 3.00 0.00 3.00 CELL 2

[0130] The cells were discharged at different current values at a depthof discharge of 50% for 10 seconds. The power density of each cell wasdetermined from the relationship between the cell voltage as measured 10seconds after the discharge and the current value measured at the sametime. Tables 7 to 9 show the results. TABLE 7 CELL NO. POWER DENSITY(W/kg) INVENTION 802 CELL 1 INVENTION 912 CELL 2 INVENTION 947 CELL 3INVENTION 973 CELL 4 INVENTION 935 CELL 5 INVENTION 871 CELL 6 INVENTION832 CELL 7 INVENTION 909 CELL 8 INVENTION 927 CELL 9 INVENTION 934 CELL10 INVENTION 853 CELL 11 COMP. 735 CELL 1 COMP. 786 CELL 2

[0131] TABLE 8 CELL NO. POWER DENSITY (W/kg) INVENTION 800 CELL 12INVENTION 895 CELL 13 INVENTION 914 CELL 14 INVENTION 927 CELL 15INVENTION 899 CELL 16 INVENTION 843 CELL 17 INVENTION 810 CELL 18INVENTION 894 CELL 19 INVENTION 900 CELL 20 INVENTION 899 CELL 21INVENTION 831 CELL 22 COMP. 735 CELL 1 COMP. 786 CELL 2

[0132] TABLE 9 CELL NO. POWER DENSITY (W/kg) INVENTION 931 CELL 23 COMP.735 CELL 1 COMP. 786 CELL 2

[0133] Tables 7 and 8 reveal that invention cells 1 to 11 and 12 to 22are higher than comparative cells 1 and 2 in power density. This isattributable to the fact that these invention cells include the currentcollector plate 3 which had a two-layer structure, i.e., copper layer31, and nickel layer 32 or stainless steel layer 35, and whichsuppressed the reflection of the laser beam used for welding the plate 3to the rolled-up electrode unit 2 and was therefore reliably welded tothe edge of the negative electrode 21 to result in an improved currentcollecting efficiency.

[0134] With comparative cell 1, on the other hand, the laser beam wasreflected by the surface of the negative electrode current collectorplate of copper, failing to completely weld the plate and leading to alower current collecting efficiency.

[0135] With comparative cell 2, increased electric resistance of thenickel collector plate led to a reduced current collecting efficiency.

[0136] Invention cells 2 to 5 and 13 to 16 which are in the range of0.10 mm to 5.00 mm in the overall thickness of the negative electrodecurrent collector plate 3 are greater in power density than inventioncells 1, 6, 12 and 17 which are outside this range. This is because whenthe thickness of the plate 3 becomes smaller than 0.10 mm, the electricresistance of the collector plate 3 itself increases, consequentlyentailing a reduced current collecting efficiency, and further becauseif the thickness of the collector plate 3 is in excess of 5.00 mm, anunsatisfactory weld will result to entail a lower current collectingefficiency.

[0137] Further invention cells 4 and 8 to 10 wherein the ratio of thethickness of the nickel layer 32 to the thickness of the currentcollector plate 3 is in the range of 5% to 45% are greater in powerdensity than invention cells 7 and 11 wherein the ratio is outside thisrange. Similarly, invention cells 15 and 19 to 21 wherein the ratio ofthe thickness of the stainless steel layer 35 to the thickness of thecurrent collector plate 3 is in the range of 5% to 45% are greater inpower density than invention cells 18 and 22 wherein the ratio isoutside this range. The reason is that if the ratio of the thickness ofthe nickel layer 32 or stainless steel layer 35 is smaller than 5%, thesurface of the copper layer 31 appears immediately after the start ofwelding of the collector plate 3, resulting in increased laser beamreflectivity and insufficient welding to entail a lower currentcollecting efficiency, and that the ratio of the thickness of the nickellayer 32 or stainless steel layer 35, if in excess of 45%, increases theelectric resistance of the collector plate 3 to result in a reducedcurrent collecting efficiency.

[0138] Table 9 further reveals that invention cell 23 is higher thancomparative cells 1 and 2 in power density. This indicates that the sameeffect as above is available by using the negative electrode currentcollector plate 3 of three-layer structure wherein the stainless steellayer 39 is interposed between the nickel layer 32 and the copper layer31.

[0139] The results described indicate that the provision of thecollector plate 3 comprising the copper layer 31, and the nickel layer32 or stainless steel layer 35 affords an improved current collectingefficiency, consequently giving an increased power density. It can besaid that the thickness of the negative electrode current collectorplate 3 is preferably in the range of 0.10 mm to 5.00 mm, and that theratio of the thickness of the nickel layer 32 or stainless steel layer35 to the overall thickness of the collector plate 3 is preferably 5% to45%. It is also apparent that if the values are within these ranges, thecollector plate 3 can be composed of at least two layers.

[2] Second Embodiment

[0140]FIG. 8 shows this embodiment, i.e., a cylindrical lithium ionsecondary cell, which comprises a cylindrical battery can 1 formed byfixedly welding lids 16, 16, to opposite ends of a cylinder 15, and arolled-up electrode unit 7 encased in the can 1. A pair of positive andnegative electrode terminal assemblies 110, 110 are mounted on therespective lids 16, 16. The terminal assemblies 110 have the sameconstruction as those of the prior art. Each lid 16 is provided with agas vent valve 13 which is openable with pressure.

[0141] A current collector plate 8 is disposed at each of opposite endsof the electrode unit 7 and joined to a current collector edge 78 bylaser welding. A lead portion 85 extending from an end portion of thecollector plate 8 has an outer end joined to a flange 112 of anelectrode terminal 111 constituting the terminal assembly 110 by spotwelding, ultrasonic welding or laser welding.

[0142] Rolled-up Electrode Unit 7

[0143] As shown in FIG. 9, the rolled-up electrode unit 7 comprises apositive electrode 71 and a negative electrode 73, which are each in theform of a strip, and a striplike separator 72 sandwiched between theseelectrodes, and is prepared by rolling up these components into a spiralform. The positive electrode 71 is formed by coating opposite surfacesof a current collector 75 in the form of a strip of aluminum foil with apositive electrode active material 74 comprising a compound oxide. Thenegative electrode 73 is formed by coating opposite surfaces of acurrent collector 77 in the form of a strip of copper foil with anegative electrode active material 76 containing a carbon material. Theseparator 72 is impregnated with a nonaqueous electrolyte.

[0144] The positive electrode 71 has a portion coated with the positiveelectrode active material 74, and a portion not coated with the activematerial. The negative electrode 73 also has a portion coated with thenegative electrode active material 76, and a portion not coated with theactive material.

[0145] The positive electrode 71 and the negative electrode 73 are eachsuperposed on the separator 72 as displaced widthwise thereof toposition the uncoated portions of the positive electrode 71 and thenegative electrode 73 as projected outward beyond the respective edgesof the separator 72. The components are rolled up into a spiral form toobtain an electrode unit 7. In this rolled-up electrode unit 7, thecurrent collector edge 78 of uncoated portion of the positive electrode71 is positioned as projected outward beyond one edge of the separator72 at one of opposite ends of the electrode unit 7 in the direction ofits winding axis, and the current collector edge 78 of uncoated portionof the negative electrode 73 is positioned as projected outward beyondthe other edge of the separator 72 at the other end of the unit 7.

[0146] Current Collecting Structure

[0147]FIGS. 9 and 10 show a current collector plate 8 which comprises acircular flat platelike body 81 integrally formed with a plurality ofcircular-arc protrusions 82 extending radially thereof and projectingtoward the rolled-up electrode unit 7. The collector body 81 has acenter hole 84 and a plurality of liquid inlets 83 around the centerhole 84. The aforementioned lead portion 85, which is in the form of astrip, is integral with an end portion of the collector body 81.

[0148] Each protrusion 82 of the collector plate 8 is in the form of acircular arc, i.e., semicircular, in section orthgonal to a radial lineof the collector body 81 as seen in FIG. 11.

[0149] Other Current Collecting Structure

[0150]FIGS. 16 and 17 show a current collector plate 100 having adifferent construction. The collector plate 100 comprises a circularflat platelike body 101 integrally formed with a plurality oftrapezoidal protrusions 102 extending radially thereof and projectingtoward the rolled-up electrode unit 7.

[0151] The collector body 101 has a center hole 104 and a plurality ofliquid inlets 103 around the center hole 104. A lead portion 105 in theform of a strip is integral with an end portion of the collector body101.

[0152] The collector body 101 is further provided along its outerperiphery with a current collector pressing portion 106 projectingdownward and positioned close to each of opposite sides of theprotrusion 102 for pressing the outer end of the current collector 77 ofthe electrode unit 7 inwardly of the unit 7. The current collectorpressing portion 106 is formed by cutting and bending an outerperipheral portion of the collector body 101 to the shape of a stripmeasuring 2 mm in width X and 5 mm in length Y as shown in FIG. 17.

[0153] Fabrication Process

[0154] Prepared first are a battery can 1 and electrode terminalassemblies 110 which are shown in FIG. 8, and a rolled-up electrode unit7 and current collector plates 8 which are shown in FIG. 9. Thecollector plates 8 are then pressed against the current collector edges78 at the respective ends of the electrode unit 7 as shown in FIG. 12.

[0155] This forces each circular-arc protrusion 82 of the collectorplate 8 into the current collector edge 78 of the electrode unit 7 asshown in FIG. 13, forming a cylindrical junction between the protrusion82 and the current collector edge 78.

[0156] In this state, a laser beam is projected onto the inner surfaceof the protrusion 82 of the plate 8 for laser welding as indicated by anarrow in the drawing. Consequently, the protrusion 82 of the collectorplate 8 and the current collector edge 78 of the electrode unit 7 arejoined to each other over a large area of contact.

[0157] In the case where the current collector plate 100 shown in FIGS.18 and 19 is used, the collector plate 100 is pressed against the end ofthe rolled-up electrode unit 7, whereby the corresponding end of thecurrent collector 77 is deflected inwardly of the unit 7 by beingpressed by the current collector pressing portion 106. This shifts theposition of contact between the current collector end and the protrusion102 of the collector plate 100 also inwardly of the electrode unit 7. Onthe other hand, when the collector plate 100 is welded to the end of theelectrode unit 7 with a laser beam, the laser beam is moved, forexample, from the inner peripheral side of the plate 100 toward theouter periphery thereof along the protrusion 102 of the plate 100 asindicated in two-dot chain lines in FIG. 18 to show the path of movementof the beam spot 107. The spot 107 a as positioned most radiallyoutwardly of the collector plate 100 can be confined to an area slightlyinwardly of the radial outer end 102 a of the protrusion 102 of theplate 100 due to the deflection of the end portion of the currentcollector 77. Suppose the outermost spot 107 a is positioned at theradial outer end 102 a of the collector plate protrusion 102. The laserbeam is then partly projected outwardly of the outer periphery of thecollector plate 100, possibly melting the outermost portion of thecurrent collector 77 or separator 72 of the electrode unit 7. With thestructure shown in FIGS. 18 and 19, in contrast, the outermost spot 107a will not be positioned outside the outer periphery of the collectorplate 100. This eliminates the likelihood of the laser beam melting thecurrent collector 77 or separator 72, consequently assuring that thecollector plate 100 will be reliably welded even to the radiallyoutermost portion of the current collector 77 of the unit 7 like theother portions thereof and permitting the plate 100 to be joined to theelectrode unit 7 over an increased area to achieve an improved currentcollecting efficiency.

[0158] Assembly of Cells

[0159] Invention cells A, B, C, D, E and comparative cells F, G, H, Iwere fabricated in the following manner.

[0160] For invention cell A, a rolled-up electrode unit 7 was preparedby arranging in superposed layers a positive electrode 71 obtained bycoating an aluminum current collector 75 having a thickness of 20 μmwith a positive electrode active material 74 comprising LiCo₂, anegative electrode 73 obtained by coating a copper current collector 77having a thickness of 20 μm with a negative electrode active material 76of graphite and a separator 72 in the form of an ion-permeable finelyporous membrane of polypropylene, and rolling up these components into aspiral form. The positive electrode 71 and the negative electrode 73each had an uncoated portion of predetermined width at a widthwise endthereof.

[0161] A current collector plate 8 of aluminum was prepared whichcomprised a flat platelike body 81 having a thickness of 1 mm and aplurality of circular-arc radial protrusions 82 and formed with aplurality of liquid inlets 83 in an opening ratio of 50%. The collectorplate 8 was fitted over the positive electrode current collector edge 78of the electrode unit 7 and pressed thereagainst with a jig from above.The circular-arc protrusions 82 of the collector plate 8 were 1 mm inwall thickness T and 1.2 mm in inside radius R.

[0162] In this state, a laser beam was projected onto the inner surfaceof each protrusion 82 of the plate 8 as shown in FIG. 13 to weld theouter peripheral surface of the protrusion 82 to the current collectoredge 78. A current collecting structure for the positive electrode wasthen made by welding the base end of an aluminum lead piece, 1 mm inthickness, to the surface of the collector plate 8 with a laser beam,and similarly welding the outer end of the lead piece to the rear faceof an aluminum electrode terminal. A negative electrode currentcollecting structure was prepared in the same manner as above exceptthat the electrode terminal, current collector plate and lead piece usedwere made from nickel.

[0163] The rolled-up electrode unit 7 was thereafter encased in acylinder 15, and a lid 16 having an electrode terminal assembly 110mounted thereon is fixedly welded to each open end of the cylinder 15.An ester-type organic electrolyte containing 1 mole/liter of LiPF₆serving as the electrolytic substance to be supported was subsequentlyplaced into the cylinder to fabricate a cell having a power capacity of180-Wh class as a component cell.

[0164] Invention cells B were assembled in the same manner as inventioncell A with the exception of using current collector plates 120 havingprotrusions 121 which were trapezoidal in section as shown in FIG. 15.Seven kinds of cells B, i.e., cells B1 to B7, were prepared which were10%, 15%, 30%, 50%, 70%, 90% and 93%, respectively, in the opening ratiogiven by liquid inlets. The furrow forming each trapezoidal protrusion121 was 1.2 mm in depth H and 1.6 mm in furrow width B at the furrowbottom.

[0165] Invention cell C was assembled in the same manner as inventioncells B except that the flat collector body was integrally formed with alead portion having the same thickness as the collector body. Theopening ratio given by the liquid inlets was 50%. The outer end of thelead portion was welded to the rear face of the electrode terminal witha laser beam.

[0166] Invention cells D, i.e., 23 kinds of invention cells D1 to D23,were assembled basically in the same manner as invention cell C exceptthat the cells were different in the shape and size of the furrowforming the trapezoidal protrusion as will be described below. The areaof openings was 50% of the overall area.

[0167] Invention cells D1 to D5 were 0.6 times, 0.8 times, 1.0 times,1.2 times and 1.6 times the laser spot diameter, respectively, in thefurrow width B at the furrow bottom. Invention cells D6 to D14 were 0.3mm, 0.5 mm, 0.8 mm, 1.2 mm, 1.6 mm, 2.0 mm, 2.5 mm, 3.0 mm and 3.5 mm,respectively, in furrow depth H. Further invention cells D15 to D23 were0.05 mm, 0.10 mm, 0.20 mm, 0.50 mm, 1.00 mm, 1.50 mm, 2.00 mm, 2.50 mmand 3.00 mm, respectively, in the thickness T of the current collectorplate.

[0168] Invention cells D1 to D5 were 1 mm in the thickness T of thecurrent collector plate, 1.2 mm in the furrow depth H of the protrusionand 1 mm in the wall thickness S of the protrusion. Invention cells D6to D14 were 1 mm in the thickness T of the current collector plate, 1.6mm in the furrow width B of the protrusion and 1 mm in the wallthickness S of the protrusion. Invention cells D15 to D23 had aprotrusion wall thickness S which was equal to the thickness T of thecurrent collector plate, and were 1.6 mm in the furrow width B of theprotrusion and 1.2 mm in the furrow depth H of the protrusion.

[0169] Invention cell E was assembled in the same manner as inventioncell D except that the cell had current collector plates 120 as shown inFIG. 15 and measuring 1 mm in thickness T and 0.5 mm in the wallthickness S of the trapezoidal protrusion 121. The opening ratio givenby the liquid inlets was 50%. The furrow depth H of the protrusion was1.2 mm and the furrow width B at the furrow bottom of the protrusion was1.6 mm.

[0170] To fabricate comparative cell F, on the other hand, currentcollector plates 92 were prepared which comprised a flat platelike body93 having a thickness of 1 mm and four bent portions 94 as shown in FIG.24. Each collector plate 92 was placed at the current collector edge 78of a rolled-up electrode unit 7 and joined thereto by spot welding usingtwo electrode rods. A lead was joined at opposite ends thereof to thecollector plate 92 and an electrode terminal by spot welding to providea current collecting structure, and the components were assembled into acell in the same manner as above.

[0171] For comparative cell G, current collector members 95 wereprepared which had a plurality of slits 96 as shown in FIG. 25. Thecurrent collector edge 78 of a rolled-up electrode unit 7 was insertedinto the slits 96 of each collector member 95, which was joined to thecurrent collector edge 78 by laser welding. A lead was joined atopposite ends thereof to the collector member 95 and an electrodeterminal by laser welding to provide a current collecting structure, andthe components were assembled into a cell in the same manner as above.

[0172] To fabricate comparative cell H, a current collector plate 9 ofaluminum having a thickness of 1 mm and protrusions 91 V-shaped insection and having an end angle of 45° was pressed against the edge 78of a positive electrode current collector of aluminum having a thicknessof 20 μm and included in a rolled-up electrode unit as shown in FIG. 14.Each V-shaped protrusion 91 was irradiated with a laser beam in thisstate for laser welding. An aluminum lead, 1 mm in thickness, wasthereafter joined at opposite ends thereof to the collector plate 9 andan electrode terminal to provide a current collecting structure for thepositive electrode.

[0173] A negative electrode current collecting structure was prepared inthe same manner as the structure for the positive electrode except thatthe electrode terminal, lead and current collector plate were made fromnickel.

[0174] Invention cells I were assembled in the same manner as inventioncell D with the exception of using current collector plates 100 havingprotrusion 102 of trapezoidal section as seen in FIGS. 16 and 17. Eachplate 100 was 1 mm in thickness T, 1.2 mm in the furrow depth H of theprotrusion, 0.5 mm in the wall thickness S of the protrusion, 1.6 mm inthe furrow width B of the protrusion, 50% in the opening ratio given bythe liquid inlets 103, 2 mm in the width X of the current collectorpressing portion 106 and 5 mm in length Y thereof. Thus, six kinds ofinvention cells I1 to I6 were fabricated which were 15°, 30°, 40°, 45°,60° and 80°, respectively, in the angle θ made by the current collectorpressing face of the current collector pressing portion 106 and thesurface of the flat platelike body 101 of the collector plate 100 asshown in FIG. 19.

[0175] Test

[0176] The cells described above were tested for performance for thecomparison of power characteristics.

[0177] Tables 10 to 12 collectively show the constructions of the cellsand the measurement of powers. TABLE 10 COLLECTOR PROTRUSION PRO-OPENING SECTIONAL FURROW FURROW PLATE WALL POWER TRU- RATIO INTEGRALSHAPE OF WIDTH B DEPTH H THICKNESS THICKNESS DENSITY SION (%) LEADPROTRUSION (mm) (mm) (mm) (mm) (W/kg) CELL A ◯ 50 X SEMICIRCULAR — 1.21.00 1.00 590 CELL B1 ◯ 10 X TRAPEZOIDAL 1.6 1.2 1.00 1.00 599 B2 ◯ 15 XTRAPEZOIDAL 1.6 1.2 1.00 1.00 599 B3 ◯ 30 X TRAPEZOIDAL 1.6 1.2 1.001.00 598 B4 ◯ 50 X TRAPEZOIDAL 1.6 1.2 1.00 1.00 598 B5 ◯ 70 XTRAPEZOIDAL 1.6 1.2 1.00 1.00 595 B6 ◯ 90 X TRAPEZOIDAL 1.6 1.2 1.001.00 593 B7 ◯ 93 X TRAPEZOIDAL 1.6 1.2 1.00 1.00 590 CELL C ◯ 50 ◯TRAPEZOIDAL 1.6 1.2 1.00 1.00 611 CELL D1 ◯ 50 ◯ TRAPEZOIDAL 0.6 1.21.00 1.00 600 D2 ◯ 50 ◯ TRAPEZOIDAL 0.8 1.2 1.00 1.00 606 D3 ◯ 50 ◯TRAPEZOIDAL 1.0 1.2 1.00 1.00 608 D4 ◯ 50 ◯ TRAPEZOIDAL 1.2 1.2 1.001.00 610 D5 ◯ 50 ◯ TRAPEZOIDAL 1.6 1.2 1.00 1.00 611 D6 ◯ 50 ◯TRAPEZOIDAL 1.6 0.3 1.00 1.00 601 D7 ◯ 50 ◯ TRAPEZOIDAL 1.6 0.5 1.001.00 607 D8 ◯ 50 ◯ TRAPEZOIDAL 1.6 0.8 1.00 1.00 609 D9 ◯ 50 ◯TRAPEZOIDAL 1.6 1.2 1.00 1.00 611 D10 ◯ 50 ◯ TRAPEZOIDAL 1.6 1.6 1.001.00 613 D11 ◯ 50 ◯ TRAPEZOIDAL 1.6 2.0 1.00 1.00 615 D12 ◯ 50 ◯TRAPEZOIDAL 1.6 2.5 1.00 1.00 616 D13 ◯ 50 ◯ TRAPEZOIDAL 1.6 3.0 1.001.00 616 D14 ◯ 50 ◯ TRAPEZOIDAL 1.6 3.5 1.00 1.00 616

[0178] TABLE 11 COLLECTOR PROTRUSION PRO- SECTIONAL FURROW FURROW PLATEWALL POWER TRU- OPENING INTEGRAL SHAPE OF WIDTH DEPTH H THICKNESSTHICKNESS DENSITY SION RATIO(%) LEAD PROTRUSION B(mm) (mm) (mm) (mm)(W/kg) D15 ◯ 50 ◯ TRAPEZOIDAL 1.6 1.2 0.05 0.05 590 D16 ◯ 50 ◯TRAPEZOIDAL 1.6 1.2 0.10 0.10 597 D17 ◯ 50 ◯ TRAPEZOIDAL 1.6 1.2 0.200.20 602 D18 ◯ 50 ◯ TRAPEZOIDAL 1.6 1.2 0.50 0.50 608 D19 ◯ 50 ◯TRAPEZOIDAL 1.6 1.2 1.00 1.00 611 D20 ◯ 50 ◯ TRAPEZOIDAL 1.6 1.2 1.501.50 614 D21 ◯ 50 ◯ TRAPEZOIDAL 1.6 1.2 2.00 2.00 616 D22 ◯ 50 ◯TRAPEZOIDAL 1.6 1.2 2.50 2.50 616 D23 ◯ 50 ◯ TRAPEZOIDAL 1.6 1.2 3.003.00 616 CELL E ◯ 50 ◯ TRAPEZOIDAL 1.6 1.2 1.00 0.50 620 CELL F COMP.540 CELL G COMP. 560 CELL H COMP. ZERO X V-SHAPED — 1.2 1.00 1.00 570

[0179] TABLE 12 OPEN- COLLECTOR PROTRUSION PRO- ING SECTIONAL FURROWFURROW PLATE WALL ANGLE POWER TRU- RATIO INTEGGRAL SHAPE OF WIDTH BDEPTH H THICKNESS THICKNESS θ DENSITY SION (%) LEAD PROTRUSION (mm) (mm)(mm) (mm) (°) (W/kg) I1 ◯ 50 ◯ RAPEOIDAL 1.6 1.2 1.00 0.50 15 622 I2 ◯50 ◯ RAPEOIDAL 1.6 1.2 1.00 0.50 30 634 I3 ◯ 50 ◯ RAPEOIDAL 1.6 1.2 1.000.50 40 638 I4 ◯ 50 ◯ RAPEOIDAL 1.6 1.2 1.00 0.50 45 636 I5 ◯ 50 ◯RAPEOIDAL 1.6 1.2 1.00 0.50 60 625 I6 ◯ 50 ◯ RAPEOIDAL 1.6 1.2 1.00 0.5080 623 ELL E ◯ 50 ◯ RAPEOIDAL 1.6 1.2 1.00 0.50 —(0) 620

[0180] Comparison of Power Characteristics of Invention Cell A andComparative Cells F, G, H

[0181] For an power characteristics test, invention cell A andcomparative cells F, G, H were charged at 0.125 C to 4.1 V, thendischarged at 0.5 C to a depth of discharge of 40% and thereafterchecked for power characteristics at a current value of 4 C for adischarge period of 10 seconds. Table 13 shows the result. The powerdensity was determined by calculating the power value based on thevoltage-current characteristics under the above conditions and dividingthe result by the weight of the cell.

[0182] Incidentally, the conditions for laser welding for thefabrication of invention cell A were: laser power of 400 W, pulsefrequency of 15 Hz and laser beam spot diameter D of 1 mm. TABLE 13POWER DENSITY (W/kg) CELL A (INVENTION CELL) 590 CELL F (COMP. CELL) 540CELL G (COMP. CELL) 560 CELL H (COMP. CELL) 570

[0183] The result given in Table 13 reveals that invention cell A ishigher than comparative cell F in power characteristics. This appearsattributable to an increase in the internal resistance of comparativecell F resulting from small areas of welds produced by spot weldingsince the current collectors are as thin as 20 μm.

[0184] Comparative cell G had a higher power than comparative cell F butis inferior to invention cell A in power. This is attributable to thefeature of invention cell A wherein the current was collected by fourradial circular-arc protrusions 82 and which therefore exhibited adiminished current distribution, whereas comparative cell G had astructure for collecting the current from a portion, in circumferentialdirection, of the electrode unit and therefore exhibited a greatercurrent distribution than invention cell A during high-rate dischargealthough the area of contact between the current collector and thecurrent collector member was greater than in invention cell A.

[0185] Furthermore, comparative cell G requires work for inserting thecurrent collector into the slits of the current collector member, hencea complex procedure, whereas in the case of invention cell A, thecurrent collector plate needs only to be pressed against the currentcollector edge to ensure a simplified welding step.

[0186] In power, comparative cell H is higher than comparative cell Gbut lower than invention cell A. Although comparative cell H, likeinvention cell A, is adapted to collect the current from the entirecurrent collector of the rolled-up electrode unit, the protrusion 91 isV-shaped in section as seen in FIG. 14, so that the width W′ of thejunction of the protrusion 91 and the current collector edge 78 issmaller than the width W of the junction of the circular-arc protrusion82 and the current collector edge 78 notwithstanding that the protrusion82 is the same as the protrusion 91 in depth and width. The differencein power is thought attributable to the smaller width W′ which resultedin a smaller contact area.

[0187] Comparison of Power Characteristics of Invention Cells A and B4

[0188] Invention cell A and invention cell B4 were checked for thecomparison of power characteristics in the case where the currentcollector plates thereof were welded under the same conditions, i.e.,400 W in laser power, and 15 Hz in pulse frequency. Table 14 shows theresult. For an power characteristics test, the cells were charged at0.125 C to 4.1 V, then discharged at 0.5 C to a depth of discharge of40% and thereafter checked for power at a current value of 4 C for adischarge period of 10 seconds. TABLE 14 POWER DENSITY (W/kg) CELL A(INVENTION CELL) 590 CELL B4 (INVENTION CELL) 598

[0189] The result of Table 14 reveals that invention cell B4 is superiorto invention cell A in power characteristics, presumably because thetrapezoidal protrusion 102 of cell B4 is greater than the circular-arcprotrusion 82 of cell A in the area of contact of the protrusion withthe current collector edge 78, and further because the portion of thecell B4 to be irradiated with the laser beam is flat over a wider area,permitting the laser beam energy to act more effectively to produce aweld over a sufficient junction area.

[0190] Comparison of Electrolyte Impregnation Time of Invention CellsB1-B7

[0191] Next, invention cells B1 to B7 were tested for impregnation withthe electrolyte in the following manner and checked for the time takenfor the rolled-up electrode unit to be impregnated with the electrolyte.

[0192] For each of invention cells B1 to B7, the rolled-up electrodeunit having the current collector plates attached thereto was checkedfor weight and then placed into a container of SUS within a dry boxhaving an argon gas atmosphere. The container was filled with theelectrolyte and subjected to a pressure of 5 kg/cm². The electrode unitwas withdrawn from the container every 10 minutes and checked for weightto measure the time taken for a predetermined amount of electrolyte toimpregnate the electrode unit. Table 15 shows the result. TABLE 15 CELLB1 B2 B3 B4 B5 B6 B7 OPENING RATIO (%) 10 15 30 50 70 90 93 IMPREGNATIONTIME (min.) 60 40 30 20 20 20 20

[0193] The result of Table 15 indicates that if the opening area issmaller than 15%, the time taken for the electrolyte to completelyimpregnate the electrode unit greatly increases.

[0194] Next, cells were fabricated using other rolled-up electrode unitshaving the same specifications as these electrode units, and tested forpower characteristics for comparison. The result is given in Table 16.For testing, the cells were charged at 0.125 C to 4.1 V, then dischargedat 0.5 C to a depth of discharge of 40% and checked for power at acurrent value of 4 C for a discharge period of 10 seconds. TABLE 16 CELLB1 B2 B3 B4 B5 B6 B7 OPENING RATIO(%) 10 15 30 50 70 90 93 OUTPUTDENSITY 599 599 598 598 595 593 590 (W/kg)

[0195] The result of Table 16 reveals that the power characteristicsmarkedly become impaired if the opening ratio of the current collectorplate given by the liquid inlets thereof exceeds 90%. Presumably, thereason is that almost entire area of the current collector plate otherthan the protrusions then serves to provide openings to result in alower current collecting efficiency.

[0196] The result described above indicates that the opening ratio ofthe current collector plate given by the liquid inlets is preferably inthe range of 15% to 90%.

[0197] Comparison of Power Characteristics of Invention Cells B4 and C

[0198] Invention cell B4 and invention cell C were charged at 0.125 C to4.1 V, then discharged at 0.5 C to a depth of discharge of 40% andthereafter checked for power at a current value of 4 C for a dischargeperiod of 10 seconds. Table 17 shows the result. TABLE 17 POWERDENSITY(W/kg) CELL B4 (INVENTION CELL) 598 CELL C (INVENTION CELL) 611

[0199] The result of Table 17 reveals that invention cell C is superiorto invention cell B4 in power characteristics. Presumably, the reason isthat the lead of the current collector is formed integrally therewith ininvention cell C, whereas the lead is welded to the current collectorplate in invention cell B4 and therefore has increased contactresistance, leading to the difference in power characteristics.

[0200] Comparison of Power Characteristics of Invention Cells D1-D5Invention cells D1 to D5 were checked for the comparison of powercharacteristics in the case where the current collector plates thereofwere welded under the same conditions, i.e., 400 W in laser power, and15 Hz in pulse frequency. Table 18 shows the result. The laser beam was1 mm in spot diameter. For an power characteristics test, the cells werecharged at 0.125 C to 4.1 V, then discharged at 0.5 C to a depth ofdischarge of 40% and checked for power at a current value of 4 C for adischarge period of 10 seconds. TABLE 18 CELL D1 D2 D3 D4 D5 FURROWWIDTH/SPOT DIAM. 0.6 0.8 1.0 1.2 1.6 POWER DENSITY(W/kg) 600 606 608 610611

[0201] The result of Table 18 reveals that when the furrow width at thebottom of the furrow forming the collector plate protrusion is smallerthan 0.8 times the spot diameter D of the laser beam, the power greatlyreduces. Presumably, the reason is that if the furrow width of theprotrusion is smaller than 0.8 times the laser beam spot diameter D, thelaser beam is projected onto opposite ends of the protrusion, i.e.,regions not to be welded to the current collector edge, whereby theenergy of the laser beam to be used effectively for welding isdiminished, failing to fully melt the portions to be welded andconsequently reducing the area of contact between the collector plateand the current collector edge to result in an impaired currentcollecting efficiency.

[0202] Accordingly, it is desired that the furrow width of the collectorplate protrusion be at least 0.8 times the spot diameter D of the laserbeam.

[0203] Comparison of Power Characteristics of Invention Cells D6-D14

[0204] Invention cells D6 to D14 were checked for the comparison ofpower characteristics in the case where the current collector platesthereof were welded under the same conditions, i.e., 400 W in laserpower, and 15 Hz in pulse frequency. Table 19 shows the result. Thelaser beam was 1 mm in spot diameter. For an power characteristics test,the cells were charged at 0.125 C to 4.1 V, then discharged at 0.5 C toa depth of discharge of 40% and checked for power at a current value of4 C for a discharge period of 10 seconds. CELL D6 D7 D8 D9 D10 D11 D12D13 D14 FUR- 0.3 0.5 0.8 1.2 1.6 2.0 2.5 3.0 3.5 ROW DEPTH (mm) POW- 601607 609 611 613 615 616 616 616 ER DEN- SITY (W/kg)

[0205] The result of Table 19 reveals that when the furrow depth of theprotrusion is smaller than 0.5 mm, the power greatly reduces.Presumably, the reason is that if the furrow depth of the protrusion issmaller than 0.5 mm, the protrusion will not fully wedge into all turnsof the current collector in the case where the edge portions of turns ofthe current collector of the rolled-up electrode unit are not positionedin a plane, consequently resulting in a decreased area of contact toentail a lower current collecting efficiency.

[0206] Further the power characteristics remain unaltered even if thefurrow depth of the protrusion is greater than 3 mm presumably becauseeven if the furrow depth is greater than 3 mm, the effect to increasethe area of contact remains unchanged since the variations in theposition of the current collector edge of the rolled-up electrode unitare usually up to 2 mm. However, if the furrow depth of the currentcollector plate protrusion is excessively large, the collector plateoccupies a greater volume in the interior of the battery can to diminishthe volumetric energy density of the cell.

[0207] Accordingly, it is preferred that the furrow depth of thecollector plate protrusion be in the range of 0.5 mm to 3 mm.

[0208] Comparison of Power Characteristics of Invention Cells D15-D23

[0209] Invention cells D15 to D23 were checked for power characteristicsin the case where the current collector plates thereof were welded underthe same conditions, i.e., 400 W in laser power, and 15 Hz in pulsefrequency. Table 20 shows the result. For a power characteristics test,the cells were charged at 0.125 C to 4.1 V, then discharged at 0.5 C toa depth of discharge of 40% and checked for power at a current value of4 C for a discharge period of 10 seconds. TABLE 20 CELL D15 D16 D17 D18D19 D20 D21 D22 D23 THICKNESS (mm) 0.05 0.1 0.2 0.5 1.0 1.5 2.0 2.5 3.0POWER DENSITY 590 597 602 608 611 614 616 616 616 (W/kg)

[0210] The result of Table 20 reveals that when the thickness of thecurrent collector plate is smaller than 0.1 mm, the power greatlyreduces. Presumably, the reason is that when having a thickness smallerthan 0.1 mm, the collector plate has increased electric resistance toexhibit an impaired current collecting efficiency. However, even if thethickness of the collector plate is made greater than 2 mm, the effectto improve the current collecting efficiency levels off, while the leadportion projecting from the collector plate then becomes less amenableto working such as bending.

[0211] Accordingly, it is desirable that the thickness of the currentcollector plate be in the range of 0.1 mm to 2 mm.

[0212] Comparison of Power Characteristics of Invention Cells D5 and E

[0213] Invention cells D5 and E were checked for power characteristicsin the case where the current collector plates thereof were welded underthe same conditions, i.e., 350 W in laser power, and 15 Hz in pulsefrequency. Table 21 shows the result. For an power characteristics test,the cells were charged at 0.125 C to 4.1 V, then discharged at 0.5 C toa depth of discharge of 40% and checked for power at a current value of4 C for a discharge period of 10 seconds. TABLE 21 POWER DENSITY(W/kg)CELL D5 (INVENTION CELL) 611 CELL E (INVENTION CELL) 620

[0214] The result of Table 21 reveals that invention cell E is superiorto invention cell D5 in power characteristics. The reason is thatalthough there is no difference between cells E and D5 in the electricresistance of the current collector plate since the collector plates ofthese cells have the same thickness, the protrusion of the cell E to beirradiated with the laser beam is smaller in wall thickness, permittinga smaller quantity of laser energy to melt the junction to be welded andconsequently realizing welding over a large contact area to result in ahigher current collecting efficiency.

[0215] Study on Radius R of Circular-Arc Protrusions in Invention Cell A

[0216] Six kinds of cells were fabricated which had the sameconstruction as invention cell A except that the cells were givenvarying values of 0.2 mm, 0.4 mm, 0.6 mm, 1.0 mm, 1.2 mm and 1.6 mm,respectively, for the inside radius R of the circular-arc protrusion 82of the current collector plate 8. The current collector plates 8 of thecells were 1 mm in the thickness of the flat platelike body 81, 1 mm inwall thickness of the circular-arc protrusion 82 and 1.2 mm in thefurrow depth of the protrusion 82. The current collector plates 8 of thecells were welded under the same conditions, i.e., 400 W in laser power,and 15 Hz in pulse frequency. To test the cells for powercharacteristics, the cells were charged at 0.125 C to 4.1 V, thendischarged at 0.5 C to a depth of discharge of 40% and checked for powercharacteristics at a current value of 4 C for a discharge period of 10seconds. Table 22 shows the result. TABLE 22 FURROW RADIUS (mm)  0.2 0.4  0.6  1.0  1.2  1.6 [RADIUS/SPOT [0.2] [0.4] [0.6] [1.0] [1.2][1.6] DIAM.] POWER DENSITY 580 585 586 588 590 591 (W/kg)

[0217] The result of Table 22 indicates that excellent powercharacteristics are available when the radius R of the circular-arcprotrusion 82 of the current collector plate 8 is at least 0.4 times thespot diameter D of the laser beam. Presumably, the reason is that if theradius R of the protrusion 82 is smaller than 0.4 times the laser beamspot diameter D, the laser beam is projected onto opposite ends of theprotrusion 82, i.e., regions not to be welded to the current collectoredge 78, whereby the energy of the laser beam to be used effectively forwelding is diminished, failing to fully melt the portions to be weldedand consequently reducing the area of contact between the collectorplate and the current collector edge to result in an impaired currentcollecting efficiency.

[0218] Accordingly, it is desired that the radius R of circular-arcprotrusion 82 of the current collector plate 8 be at least 0.4 times thespot diameter D of the laser beam.

[0219] Study on Angle θ Made by Current collector Pressing Face andCurrent Collector Body Surface in Invention Cells I

[0220] Invention cells I1 to I6 and invention cell E (wherein the angleθ is 0°) were tested for power characteristics. The current collectorplates 100 of the cells were welded under the same conditions, i.e., 400W in laser power, and 15 Hz in pulse frequency. For testing, the cellswere charged at 0.125 C to 4.1 V, then discharged at 0.5 C to a depth ofdischarge of 40% and checked for power characteristics at a currentvalue of 4 C for a discharge period of 10 seconds. Table 23 shows theresult. TABLE 23 CELL E I 1 I 2 I 3 I 4 I 5 I 6 ANGLE θ (°) — (0) 15 3040 45 60 80 POWER DENSITY 620 622 634 638 636 625 623 (W/kg)

[0221] The result of Table 23 indicates that invention cells I1 to I6wherein the current collector pressing portions 106 are formed exhibit ahigher power density than invention cell E (wherein the angle θ is 0°).Presumably, the reason is that the current collector pressing portion106 deflects the end portion of the current collector 77 inwardly of theelectrode unit 7 by pressing the end portion, whereby the position ofcontact of the collector plate protrusion 102 with the current collectoris shifted also inwardly of the unit 7, consequently permitting thecurrent collector portion positioned at the outer periphery of theelectrode unit 7 to be welded like other current collector portions andensuring a large junction area to achieve an improved current collectingefficiency.

[0222] It will also be understood that more excellent powercharacteristics are available when the angle θ is at least 30° to notgreater than 45°. This is because if the angle θ is smaller than 30°,the end portion of the current collector 77 of the rolled-up electrode 7will not be fully deflected inward, and further because if the angle θis greater than 45°, the current collector pressing portion 106 will beforced into the end portion of the rolled-up electrode unit 7, failingto fully deflect the end portion of the current collector 77 inward.Resulting in either case is only a small inward shift in the position ofcontact between the current collector end portion of the electrode unit7 and the collector plate protrusion 102, so that a sufficiently largearea of junction is not available. Accordingly, the angle θ to be madeby the current collector pressing face of the current collector pressingportion 106 and the surface of flat platelike body 101 of the currentcollector plate is preferably at least 30° to not greater than 45°.

[0223] The cells of the present invention are not limited to theforegoing embodiments in construction but can be modified variouslywithin the technical scope set forth in the appended claims. Forexample, ferritic stainless steel or martensitic stainless steel is alsousable as the material for the metal layer of the negative electrodecurrent collector plate 3. Although the laser beam is used for weldingthe current collector plate according to the embodiments described, thismethod of welding is not limitative but an electron beam is also usablefor welding. The present invention can be embodied not only as lithiumion secondary cells but as a wide variety of nonaqueous electrolytesecondary cells.

What is claimed is:
 1. A nonaqueous electrolyte secondary cellcomprising a rolled-up electrode unit encased in a battery can andcomprising as superposed in layers a positive electrode and a negativeelectrode, each in the form of a strip, and a separator in the form of astrip interposed between the electrodes and impregnated with anonaqueous electrolyte, each of the positive electrode and the negativeelectrode being formed by coating a current collector in the form of astrip with an active material, and the positive electrode, negativeelectrode and separator being rolled up into a spiral form to form therolled-up electrode unit; the cell being adapted to deliver electricpower generated by the electrode unit to the outside via a pair ofelectrode terminals; the nonaqueous electrolyte secondary cell beingcharacterized in that the current collector of at least one of thepositive electrode and the negative electrode has an uncoated projectingedge at at least one of axially opposite ends of the rolled-up electrodeunit, a current collector plate pressing against the rolled-up electrodeunit and joined to said uncoated projecting edge, said currentcollecting plate having a plurality of protrusions formed therein in adirection perpendicular to the axis of the rolled-up electrode unit andprotruding in the axial direction of the rolled-up electrode unit, eachprotrusion being shaped to have a circular-arc cross section in adirection perpendicular to the axis of the rolled-up electrode unit or apolygonal cross section with at least four corners in a directionperpendicular to the axis of the rolled-up electrode unit, and eachprotrusion protruding in a direction toward said uncoated projectingedge of the rolled-up electrode unit and having one surface and an othersurface which is the reverse of said one surface, said one surface beingopposed to said uncoated projecting edge, said other surface beingrecessed in a direction toward said uncoated projecting edge and formedby a recessed surface of the protrusion, the collector plate beingwelded to said uncoated projecting edge by irradiating the recessedsurface of said plurality of protrusions with a laser beam or anelectron beam with said plurality of protrusions forced into saiduncoated projecting edge, and being connected to one of the electrodeterminals.
 2. A nonaqueous electrolyte secondary cell according to claim1 wherein the current collector plate comprises a flat platelike bodyformed with one or a plurality of openings among the protrusions whichfunction as liquid inlets, and the opening area provided by the openingsis at least 15% of the flat area of the body.
 3. A nonaqueouselectrolyte secondary cell according to claim 1 wherein the currentcollector plate is provided with a lead portion in the form of a strip,the lead portion having an outer end connected to the electrodeterminal.